Viruses may infect cells to take over the host cell machinery and produce new viral particles via transcription and translation processes. Interception of either of these processes, including pre- and post translation events, may cripple virus propagation.
Since the discovery of non-nucleoside reverse transcriptase inhibitors (NNRTI), and protease inhibitors (PI), nonnucleoside organic molecules have been designed to combat viral infections by inhibiting a variety of targets responsible for viral replication. Examples of such targets include reverse transcriptases, DNA polymerases, viral proteases (e.g., serine, cysteine, aspartyl, metalloproteases), integrases, helicases, fusion proteins, chemokines (CCR5, CXCR4), and chemokine receptors. For example, there are drugs which may prevent fusion of the viral envelope with the cell membrane and therefore inhibit the entry of viruses, such as human immunodeficiency virus (HIV), into the cell. Also, other drugs may act at the late stage of the viral replication cycle to prevent propagation of virus that is already in the cell.
Smallpox is a member of the highly homologous orthopox family of viruses. As of the 1990s, it was belileved that smallpox virus was no longer a health concern as the last known case of smallpox had occurred in 1977. Also, universal vaccination programs in the U.S. were discontinued in 1972 because the risk of complications from the vaccine was actually greater than the risk of being infected with the disease. Recently, however, cases of smallpox have been documented. In addition, due to the highly homologous nature of the orthopox family, therapeutics developed against smallpox are also potential candidate therapies for related viruses that continue to plague society such as monkeypox, a virus that recently reemerged in the Africa and spread to the US through exotic animals, and mulluscipox virus, which results in a common cutaneous infection that may be problematic with immunocompromised individuals. Thus, there is a renewed interest in developing antiviral agents to treat orthopox viruses, and more particularly, smallpox.
A wide spectrum of antiviral agents have been investigated using different strains of variola as well as other orthopoxviruses (Baker, et al., Antiviral research, 57, 13, 2003). Among antiviral compounds found to be useful were cidofovir (DNA polymerase), ribavrin and tiazofurin (IMP dehydrogenase), C-ca3-ADO, and C3-Npc-A (SAH hydrolase). Also, HPMPC (Cidofovir), a DNA Polymerase Inhibitor for the treatment of CMV retinitis in AIDS patients, may have therapeutic potential for treatment of various other herpes viruses, as well as polyomavirus, papillomavirus, adenovirus as well as poxvirus. For example, in vitro evidence demonstrates that HPMPC may be active against all poxviruses studied to date as well as vaccinia and cowpox virus infections. However, treatment with HPMPC is only currently available in either topical or intravenous forms. Also, side effects, including significant nephrotoxicity, may result.
While useful anti-viral compounds have been identified, viruses can rapidly acquire resistance to drugs. Thus, new anti-viral agents are needed that can be used alone or in a cocktail of drugs where the cocktail can cripple a virus by hitting a multitude of targets.